Sun shines on solar power steam turbine generators

21st February 2013

It is no exaggeration to say that solar power is booming. With the exception of maybe wind power, no other form of power generation has the same ability to capture the imagination of the public, the hearts of engineers, the conscience of environmentalists and the minds of politicians and economists!

A year ago, Siemens was using this forum to describe its first industrial steam turbine delivery for a solarthermal application, and advocating the new technology for the new era. In the space of a year, Siemens now has firm orders for 15 steam turbine generators for similar solar applications, with 18 orders currently being processed and slotted, and a first order for the next stage - the ISCCS or Integrated Solar Combined Cycle System.

The first plant -Nevada Solar One in Boulder City, USA, owned by Solargenix (Acciona) - is now commissioned, handed over to the customer, and producing power for the state of Nevada. The configuration has since become the template for the Siemens power generation solution for CSPP's or Concentrated Solar Power Plant.

The pioneering plant in Nevada, USA - the first solar plant there for 15 years - was followed by a wave of orders for solar developments in southern Spain, where the current legislation is extremely favourable to solar power investment. The Spanish government has namely granted a price surplus for solar-produced power valid for 25 years. This applies only to units under 50MW, which explains why the orders awarded to Siemens have been for multiple units of 50-MW steam-turbine output, varying from one to eight units. Many of the major Spanish solar players

Because the high investment cost for a CSPP with limited operational hours demands a high-efficiency perspective right from the beginning, high demands are imposed on the steam turbine used in the process. For this reason, Siemens has co-operated closely with its customers to develop and fine-tune the SST-700RH (reheat) steam turbine, now optimized for solar steam cycles and capable of generating up to 170 MW in CSPP applications. This highly efficient turbine with its high-speed, high-pressure module enables a smaller solar mirror collector field with associated reduction in investment cost; in addition, the surplus heat can be stored in large storage tanks and used to extend the running hours of the steam turbine.

When focusing on annual power production, the short startup and stop times the turbine can provide are of great benefit to the CSPP plant owner. Daily cycling and low temperature require special attention. The SST-700RH, with its low-mass rotors and casings, is ideal for daily cycling and has a low minimum load, enabling maximum running hours per day for plants without heat storage. The cycle has also been optimized to stand still at night. The SST-700RH uses high quality materials especially chosen for long and trouble-free operation in a solar plant, bearing in mind the potential wear and tear of the special cycle conditions.

IFor performance and emissions reductions, parabolic troughs can be effectively integrated with a conventional combined cycle plant, as well as a steam cycle plant.

The most recent development has been of ISCSS, integrated solar combined cycle system, resulting in a bigger plant. Siemens has recently contracted to supply an 81-MW SST-900 industrial steam turbine to such a plant at Hassi R'Mel, in Algeria, Northern Africa. This is another industrial steam turbine from the Siemens portfolio, a single-casing high-pressure non-reheat unit, suited to demands of the combined cycle. It will be used in combination with two 47-MW SGT-800 gas turbines, also from the Siemens industrial turbine manufacturing facility in Finspong, Sweden.

The dynamic order situation is partly a question of success breeding success. The expansion of solarthermal power plants has been unusually rapid. And as Siemens had already created its footprint in this market, it was natural that customers should consider the Siemens solution to be the technical benchmark, and Siemens to be the knowledge-owner and technology-leader. An experienced and renowned supplier with a global support network not only minimizes risk, but provides the more bankable solutions.

The integrated solar-combined cycle system (ISCCS) calls for part of the heat recovery steam generator (HRSG) to be either replaced or paralleled by equipment serviced by solar thermal energy to supplement turbine exhaust gases. This approach increases thermal energy input and produces more electrical output. The system design may be thermodynamically optimized by having the exhaust gas and/or the solarthermal energy yield its heat at points in the steam cycle where local heat transfer fluid temperatures are compatible with the local steam temperature, ie, all heat transfer processes take place at reasonably small temperature differences. This design philosophy optimally integrates the solar heat source into the combined cycle HRSG. The integration maximizes operation efficiency even though solar energy intensity varies according to the weather and time of day. Peak thermal-to-electric efficiency can exceed 70 per ceny for an ISCCS plant compared to 50-55 per cent for a conventional gas-fired combined cycle plant.

To increase the share of renewable energies in the Algerian national power mix, an ISCCS plant will be constructed in Hassi R'Mel in northern Algeria, comprising a conventional 150-MW combined cycle and a solar field with a nominal thermal power of 95 MWth. It is one of the two first such plants in the world, the other being at Aïn Béni Mathar in north-eastern Morocco.The goal of the project is to integrate the solar thermal technology in a conventional power plant in order to reduce the cost and facilitate the deployment of renewable energies in new industrializing countries. The project is being promoted by Solar Power Plant One (SPP1), a specially formed Abener and NEAL joint venture, who will operate the plant for a period of 25 years. The Algerian state society, Sonatrach, will buy all of the power produced, some of which may be exported.

This project will use the heat generated in the steam turbine that simultan¬eously utilises the waste heat from the gas turbine for electricity. This configuration is doubly effective. On the one hand, it minimizes the investment associated with the solar field by sharing components with the combined cycle. It also reduces the CO2 emissions associated with a conventional plant. Solarthermal technology undoubtedly has a large global potential. Where there is sun there is heat, where there is heat, there is power - clean and renewable power. Different solutions are coming to the fore to provide the amount of power needed, depending on the physical circumstances. The proven high quality, reliability and flexibility of Siemens turbine technology, as well as the early experience gained in the field, are guarantees of participation in that expanding future.